Research is proposed to increase our understanding of the mammalian fertilization process by paying special attention to the comparative ability of sperm cells to fertilize ova under defined in vitro conditions. Initial efforts will be devoted to further defining requirements for rabbit sperm capacitation in vitro with the goal of improving the rapidity with which penetration of ova takes place following in vitro insemination, to approximate that obtained by sperm optimally capacitated in vivo. A repeatable procedure for in vitro sperm capacitation and in vitro fertilization will be developed in the cow in an extension of these experiments. The normalcy of fertilization and resulting embryos will be examined by morphological studies including electron microscopy and by assessing the potential for normal development following embryo culture and embryo transfers. In vitro fertilization methodology will be employed in efforts to characterize the sperm fertilizing ability of males (bucks and bulls) by homospermic and heterospermic inseminations in vitro. Results will be compared to the reproductive history (i.e., in vivo fertilizing ability) of each male.